Biliverdin reductase (BVR) is an evolutionarily conserved soluble enzyme found primarily in mammalian species. The human reductase was recently identified as a serine/threonine kinase (Kravets et al., J. Biol. Chem. 279:19916-19923 (2004); Salim et al., J. Biol. Chem. 276:10929-34 (2001)) sharing conserved catalytic domains with known serine/threonine kinases (Hunter et al., Annu. Rev. Biochem. 54:897-930 (1985); Hanks et al., Methods Enzymol. 200:38-62 (1991)). Prior to this, the enzyme was solely considered in the context of its reductase activity and conversion of the open tetrapyrrole biliverdin to bilirubin in the cytosol (Kutty et al., J. Biol. Chem. 256:3956-62 (1981); Fakhrai et al., J. Biol. Chem. 267:4023-9 (1992); Maines et al., Eur. J. Biochem. 235:372-81 (1996)). Biliverdin is the product of the isomer specific cleavage of heme (Fe-protoporphyrin IX) by heme oxygenase isozymes HO-1 and HO-2 (Maines, HEME OXYGENASE: Clinical Applications and Functions, CRC Press Inc., Boca Raton, Fla. (1992); Maines, Annu. Rev. Pharmacol. Toxicol. 37:517-54 (1997)). BVR was also found to translocate into the nucleus in cells treated with cGMP (Maines et al., J. Pharmacol. Exp. Ther. 296:1091-7 (2001)) and function as a transcription factor for AP-1 regulated genes (Kravets et al., J. Biol. Chem. 279:19916-19923 (2004); Ahmad et al., J. Biol. Chem. 277:9226-32 (2002)). Activation of c-jun and CREB/ATF-2 by human BVR was more recently reported (Kravets et al., J. Biol. Chem. 279:19916-19923 (2004)).
Protein tyrosine kinases (PTK) are a multigenic family exclusive to the higher organisms (Hunter et al., Annu. Rev. Biochem. 54:897-930 (1985); Robinson et al., Oncogene 19:5548-57 (2000)). They function in cell signaling pathways involved in growth, differentiation and mobility of cells, and in development of diseases such as diabetes and cancer.
The action of insulin as a metabolic regulator and a growth factor is PTK-dependent and is an essential step in the initiation of signaling cascade, which is the coupling of the intracellular kinase domain of the insulin receptor (“IRK”) with insulin receptor substrate (“IRS”) (Cai et al., J. Biol. Chem. 278:25323-30 (2003); Grusovin et al., Front. Biosci. 8:d620-41 (2003); Lavan et al., J. Biol. Chem. 272:21403-7 (1997); Rocchi et al., Mol. Endocrinol. 12:914-23 (1998); White et al., Curr. Top. Microbiol. Immunol. 228:179-208 (1998)). Autophosphorylation on tyrosine residues and activation of IRK that results from conformational change in the kinase, following insulin binding to the extracellular domain of the receptor, serves as a recognition signal for IRS proteins (IRS-1-IRS-7) (Myers et al., Mol. Cell. Biol. 16:4147-55 (1996); Songyang et al., Mol. Cell. Biol. 14:2777-85 (1994); White, Am. J. Physiol. Endocrinol. Metab. 283:E413-22 (2002)).
Insulin signaling is inhibited by IRS-1 serine phosphorylation. In human IRS-1, a number of serines have been identified as the important residues, including Ser307,312 and Ser616. A number of serine/threonine kinases, including JNK and PKC, are known to phosphorylate IRS-1 (Aguirre et al., J. Biol. Chem. 275:9047-54 (2000); De Fea et al., J. Biol. Chem. 272:31400-6 (1997); Jakobsen et al., J. Biol. Chem. 276:46912-6 (2001); Kim et al., Biol. Chem. 384:143-50 (2003); Lee et al., J. Biol. Chem. 278:2896-902 (2003); Liu et al., J. Biol. Chem. 276:14459-65 (2001); Ozes et al., Proc. Natl. Acad. Sci. USA 98:4640-5 (2001); Yuan et al., Science 293:1673-7 (2001)). Serine phosphorylation of IRS-1 has been considered as a mechanism for insulin resistance (Tanti et al., J. Biol. Chem. 269:6051-7 (1994)).
Despite these advances in understanding the mechanism by which insulin resistance may occur, there remains a need to identify other kinases that can regulate insulin receptor signaling via the IRS proteins. The identification of new molecular mechanisms that can be manipulated to control insulin signaling and, consequently, glucose metabolism is highly desirable.
The present invention is directed to overcoming these and other limitations in the art.